Hexagonal boron nitride (h-BN) is a material with chemical formula BN, consisting of boron and nitrogen atoms in a planar two-dimensional hexagonal structure, and because it has chemical and physical properties similar to graphite due to having a hexagonal structure similar to graphite, its physical and chemical stability is high. In an inert atmosphere, it is stable up to 3,000° C., and has as high thermal conductivity as stainless steel and high thermal shock resistance, and is less susceptible to cracks or damage during repeated fast heating to about 1,500° C. and fast cooling. Also, high-temperature lubricity and corrosion resistance is considerably high. Further, it has an extraordinarily high electrical resistance value, particularly, with a small change in electrical resistance value at high temperature, and may be thus used as an electrical insulating material in a wide temperature range, and it is characterized in that it emits ultraviolet (UV) light when an electric field is applied. In addition, boron nitride is transparent, and due to a spatial space of a hexagonal honeycomb structure of boron and nitrogen atoms connected like a net, it is very flexible. The unique structure and properties of boron nitride may be applied to an insulation of a semiconductor material and a UV generator.
Recently, with the increasing demand and interest in nano technology, many studies are being made to prepare boron nitride, for example, in nanosheet or nanotube form.
Currently, a method of producing a hexagonal boron nitride nanosheet includes a mechanical exfoliation method, a chemical vapor deposition (CVD) method (Korean Patent No. 0433322), and a boron nitride interlayer compound method (Korean Patent No. 1212717), and generally, a CVD method and a mechanical exfoliation method are used to produce a hexagonal boron nitride nanosheet. i) The mechanical method is a method that exfoliates single-layer or multilayer boron nitride from hexagonal boron nitride in a solvent through ultrasonic wave treatment, and allows simple production but has a disadvantage of being difficult to mass produce. ii) The CVD method is generally a method that deposits a catalyst metal on a substrate to form a thin metal film, flows gas including boron and nitrogen at a high temperature of 1,000° C. or more, and cools to obtain a boron nitride nanosheet formed on the metal film, and has disadvantages of a very high process temperature and being unfavorable in terms of a large area and a cost.
Generally, the catalyst metal used in the CVD method has a polycrystalline structure, and thus, a considerable amount of boundaries of small-sized grains classified by the grain boundary exist on the surface of the catalyst. The large amount of grains and grain boundaries is one of the causes of degradation of surface quality of h-BN grown thereon. Thus, a related art drawn to a method of producing a hexagonal boron nitride sheet using a CVD method, for example, Korean Patent Application Publication No. 2013-0063410, discloses technology for preparing a thin film of h-BN by which sintering and thermal treatment is performed at high temperature to induce rearrangement of atoms within a metal catalyst, so that metal catalyst has an increased grain size and consequently a similar or same crystal face, and a thin film of h-BN is prepared using such a metal catalyst in a sheet form or the metal catalyst with a polished surface while supplying a nitrogen source and a boron source in gas phase.
Further, one of the Non-Patent Literatures, Nano Lett., 2012, 12, 714-718, discloses technology for synthesis of h-BN with ammonia borane at atmospheric pressure after thermal treatment and surface modification of a copper catalyst and its use in a graphene device, Nano Lett., 2010, 10, 4134-4139 discloses technology for production of h-BN in ten or less layers using a nickel catalyst, and RCS Advances, 2012, 2, 111-115 discloses technology for growth of h-BN in ten or more layers using a nickel catalyst, however all have a drawback of non-uniform growth of h-BN over the whole area.
These related arts simply increase a size of a crystal grain using thermal treatment of a metal catalyst having a small crystal grain size and a rough surface, or just improve the surface roughness through surface grinding of a metal catalyst, and as a consequence, quality of a resulting h-BN thin film is not good and it is impossible to accurately control the number of thin films of a resulting h-BN film, which makes it difficult to produce a single-layer thin film and even a thick film having a larger thickness, and they were only able to produce a h-BN film consisting of several layers.
Recently, with the development and market creation of nano electronic devices, as the need for development of materials with improved insulation characteristics and materials applicable as a buffer layer is growing, there is a continuously and gradually increasing demand for h-BN that may act as a high-performance insulation layer, and accordingly, there is a need for development of a method of forming a h-BN thick film on a substrate and a h-BN thick film laminate produced thereby.